High temperature resistant silicon joint for the joining of ceramics

ABSTRACT

A method for the joining of ceramic pieces with a hermetically sealed joint comprising brazing a layer of joining material between the two pieces. The ceramic pieces may be aluminum nitride or other ceramics, and the pieces may be brazed with a high purity silicon or a silicon alloy under controlled atmosphere. The joint material is adapted to later withstand both the environments within a process chamber during substrate processing, and the oxygenated atmosphere which may be seen within the interior of a heater or electrostatic chuck.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/098,334 to Elliot et al., filed Dec. 31, 2014, which is herebyincorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to methods for joining together objects,and more particularly to brazing methods for joining ceramic objects.

2. Description of Related Art

Semiconductor processing and similar manufacturing processes typicallyemploy thin film deposition techniques such as Chemical Vapor Deposition(CVD), Physical Vapor Deposition (PVD), Vapor Phase Epitaxy (VPE),Reactive Ion Etching, and other processing methods. In CVD processing,as well as in other manufacturing techniques, a substrate such as asilicon wafer is secured within a processing chamber using semiconductorprocessing equipment, such as a heater or an electrostatic chuck, andexposed to the particular processing conditions of the process. Theheater or electrostatic chuck is essentially a pedestal that, inaddition to securing the substrate, can in some instances also be usedto heat the substrate.

As heaters are exposed to high operating temperatures and corrosiveprocess gasses, and because good thermal conductivity is required forgood temperature control, prior art heaters have been made from a verylimited selection of materials, such as aluminum nitride (AlN) ceramicor PBN, silicon dioxide (quartz), graphite, and various metals such asaluminum alloys, nickel alloys, stainless steel alloys, Inconel, etc.Reactive process gasses which are typically used for semiconductorprocessing, or chamber cleaning, generally react with heaters made withmetal alloys. These reactions can produce corrosive by-products andother effects which can be detrimental to the desired process results.Ceramic materials can be much more resistant to reactions with typicalprocess gasses, and to corrosion from reaction by-products. However,ceramic materials can have limited methods of fabrication due toinherent material properties, and have high manufacturing costs.

The manufacture of semiconductor processing equipment using ceramics,such as heaters and electrostatic chucks with a ceramic shaft and aceramic plate, currently involves hot pressing sub-components to partialdensity, and then again hot pressing an entire assembly until fulldensity is attained. In this type of manufacture, at least two drawbacksare seen. First, the hot pressing/sintering of a large, complex ceramicpiece requires a large physical space, and a multiplicity of sequentialsintering steps is required. Second, should a portion of the finishedpiece become damaged, or fail due to wear, there is no repair methodavailable to disassemble the large piece, likely leading to it beingscrapped. In the case of manufacture from two or more pieces which havealready been pressed to full density, there are also at least twodrawbacks. First, after the initial sintering of the major components,these components are typically joined using a liquid phase sinteringprocess to join the major components (in the case of aluminum nitride,for example), which requires high heat, high compressive force, and asignificant amount of time in a process oven capable of providing boththe high temperatures and the high compressive force. Often the highcompressive force applied to a shaft during this sintering to a plate,such as is done in the process of creating a ceramic heater, requiresthat the annular shaft walls be of thicker cross-sectional thicknessthan desired in the finished product in order to support thesecompressive forces. The shaft may then need to be machined down to afinal lesser thickness desired to keep heat flow down the shaft to aminimum. Second, should a portion of the finished piece become damaged,or fail due to wear, there is no repair method available to successfullydisassemble a large piece that has been joined in this fashion, likelyleading to it being scrapped.

An additional concern may be with regard to the repair of these piecesof semiconductor processing equipment, such as heater and electrostaticchucks with plate and shaft elements. Should a portion of a multi-pieceassembly of equipment be damaged, such as due to arcing, for example, itmay be desirable to dis-assemble the piece of equipment and re-useportions of it. These portions may retain significant financial value.With current methods of manufacturing, for example with regards toceramic heaters, there is no method available which would allow for therepair of equipment which would allow replacement of some portions andthe re-use of some portions of that equipment.

U.S. Pat. No. 8,789,743 discloses a method for joining ceramic materialswhich does address the above-mentioned drawbacks of other priorprocesses. The method includes using a high purity aluminum brazingmaterial at temperatures which result in good and complete wetting, andhermetic joints without diffusion. A limitation on these joints,however, is that the equipment made using these joints cannot be used athigher temperatures, such as 700 C-1400 C, as that is significantlyabove the solidus temperature of the aluminum braze material.

What is called for is a joining method for joining ceramic pieces whichprovides a hermetic seal, and which does nor diffuse into the ceramicsand thus allows for repairs, and which is able to withstand subsequentexposure to processes run at higher temperatures.

SUMMARY OF THE INVENTION

A method for the joining of ceramic pieces with a hermetically sealedjoint comprising brazing a layer of joining material between the twopieces. The ceramic pieces may be aluminum nitride or other ceramics,and the pieces may be brazed with a high purity silicon, or a siliconalloy, under controlled atmosphere. The joint material may be adapted tolater withstand both the environments within a process chamber duringsubstrate processing, and the oxygenated atmosphere which may be seenwithin the interior of a heater or electrostatic chuck.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a plate and shaft device used in semiconductorprocessing according to some embodiments of the present invention.

FIG. 2 is a sketch of a high temperature press and oven for a plateaccording to some embodiments of the present invention.

FIG. 3 is a sketch of a high temperature press and oven for a pluralityof plates according to some embodiments of the present invention.

FIG. 4 is a sketch of a high temperature press and oven for a plate andshaft device according to some embodiments of the present invention.

FIG. 5A is a cross-sectional view of a joint between a plate and shaftaccording to some embodiments of the present invention.

FIG. 5B is a cross-sectional view of a joint between a plate and shaftaccording to some embodiments of the present invention.

FIG. 5C is a perspective view of a shaft end with standoffs according tosome embodiments of the present invention.

FIG. 6 is a partial cross-sectional view of a plate and shaft device inuse in semiconductor manufacturing according to some embodiments of thepresent invention.

FIG. 7 is a close-up cross-sectional view of a joint between and shaftand a plate according to some embodiments of the present invention.

FIG. 8 is view of a plate and shaft device according to some embodimentsof the present invention.

FIG. 9 is an illustration of plate and shaft ready for assemblyaccording to some embodiments of the present invention.

FIG. 10 is an illustration of plate and shaft with fixturing ready forassembly according to some embodiments of the present invention.

FIG. 11 is an illustration of plate and shaft with fixturing ready forassembly according to some embodiments of the present invention.

FIG. 12 is a photograph of a ceramic assembly with a silicon jointaccording to some embodiments of the present invention.

FIG. 13 is a photograph of a ceramic assembly with a silicon jointaccording to some embodiments of the present invention.

FIG. 14 is a cross-sectional view of a multi-layer plate assemblyaccording to some embodiments of the present invention.

DETAILED DESCRIPTION

Some prior processes for the joining of ceramic materials requiredspecialized ovens, and compression presses within the ovens, in order tojoin the materials. For example, with liquid phase sintering, two piecesmay be joined together under very high temperatures and contactpressures. The high temperature liquid-phase sintering process may seetemperatures in the range of 1700 C and contact pressures in the rangeof 2500 psi.

Other prior processes may utilize diffusion of a joining layer into theceramic, and/or of the ceramic into the joining layer. In suchprocesses, a reaction at the joint area may cause changes to thematerial composition of the ceramic in the area near the joint. Thisreaction may depend upon oxygen in the atmosphere to promote thediffusion reaction.

In contrast to the aforementioned diffusion processes, joining methodsaccording to some embodiments of the present invention do not dependupon liquid phase sintering or diffusion.

In some applications where end products of joined ceramics are used,strength of the joint may not be the key design factor. In someapplications, hermeticity of the joint may be required to allow forseparation of atmospheres on either side of the joint. Also, thecomposition of the joining material may be important such that it isresistant to chemicals which the ceramic assembly end product may beexposed to. The joining material may need to be resistant to thechemicals, which otherwise might cause degeneration of the joint, andloss of the hermetic seal. The joining materials may also need to be oftypes of materials which do not negatively interfere with the processeslater supported by the finished ceramic device.

In the processing of substrates, many processes require that thesubstrate be supported by semiconductor processing equipment components,such as a heater or an electro-static chuck. These components may bemaintained at, or required to operate in, vacuum conditions, hightemperatures, thermal cycling, corrosive atmospheres, and may be damagedduring their use during semiconductor manufacturing processes orotherwise. In some aspects, these components may be comprisedsubstantially or comprised fully of a ceramic such as aluminum nitride.The manufacture of these components from such a material has involvedcostly materials, and is time and equipment intensive, resulting in avery expensive end product.

Prior methods of manufacturing components such as heaters andelectrostatic chucks using ceramic materials have required process stepswith specialized atmospheres (such as vacuum, inert, or reducingatmospheres), very high temperatures, and very high contact pressures.The contact pressures may be applied using presses, and these pressesmay be adapted to operate inside a process chamber that provides thespecialized atmospheres, such as vacuum, and high temperatures. This mayrequire specialized presses and fixturing made of refractory materials,such as graphite, within the process chamber. The cost and complexity ofthese setups may be very high. In addition, the larger the componentthat is required to be pressed, the fewer components can be put intosuch a process oven. As the duration of the processes in the processovens with presses may be measured in days, and given the large expenseassociated with both the manufacture of and the running of the processovens/presses, a reduction in the number of steps which use theseprocess ovens which provide very high temperature, special atmospheres,and very high contact pressures during the manufacture of componentswill result in great savings.

FIG. 1 illustrates an exemplary plate and shaft device 100, such as aheater, used in semiconductor processing. In some aspects, the plate andshaft device 100 is composed of a ceramic, such as aluminum nitride.Other materials, such as alumina, silicon nitride, silicon carbide orberyllium oxide, may be used. In other aspects the plate may be aluminumnitride and the shaft may be zirconia, alumina, or other ceramic. Theheater has a shaft 101 which in turn supports a plate 102. The plate 102has a top surface 103. The shaft 101 may be a hollow cylinder. The plate102 may be a flat disc. Other subcomponents may be present. In somepresent processes, the plate 102 may be manufactured individually in aninitial process involving a process oven wherein the ceramic plate isformed. FIG. 2 conceptually illustrates a process oven 120 with a press121. The plate 122 may be compressed under temperature in a fixture 123adapted to be pressed by the press 121. The formation of the plate 122would be one of many processes needed to be performed with a veryspecialized process oven. The shaft 101 may also be similarlymanufactured in a process step. The formation of the shaft 163 would beanother of the processes needed to be performed with a very specializedprocess oven. In a typical process, the plate and shaft are formed byloading of aluminum nitride powder incorporating a sintering aide suchas yttria at about 4 weight % into a mold, followed by compaction of thealuminum nitride powder into a “solid” state typically referred to as“green” ceramic, followed by a high-temperature liquid-phase sinteringprocess which densities the aluminum nitride powder into a solid ceramicbody. The high temperature liquid-phase sintering process may seetemperatures in the range of 1700 C and contact pressures in the rangeof 2500 psi. The bodies are then shaped into the required geometry bystandard grinding techniques using diamond abrasives.

There are multiple functions of the shaft: one is to providevacuum-tight electrical communication through the wall of the vacuumchamber in order to apply electrical power to heater elements as well asa variety of other electrode types which may be embedded within theheater plate. Another is to allow temperature monitoring of the heaterplate using a monitoring device such as a thermocouple, and allowingthat thermocouple to reside outside of the processing chamberenvironment in order to avoid interaction such as corrosion between thematerials of the thermocouple and the process chemicals, as well asallowing the thermocouple junction to operate in a non-vacuumenvironment for rapid response. Another function is to provide isolationof the materials used for the previously mentioned electricalcommunication from the processing environment. Materials used forelectrical communication are typically metallic, which could therebyinteract with process chemicals used in the processing environment inways which could be detrimental to the processing results, anddetrimental to the lifetime of the metallic materials used forelectrical communication.

Given the relatively flat nature of the plate, a plurality of plates 142may be formed in a single process by stacking a plurality of platemolding fixtures 143 along the axial direction of the press 141 whichresides within the process oven 140, as seen conceptually in FIG. 3. Theshafts may also be formed in a similar process using the press in theprocess oven, wherein multiple shafts are made simultaneously side byside, for example.

In the overall process of manufacturing a heater used in semiconductorprocessing both the step of forming plates and forming shafts requiresignificant commitments of time and energy. Given the cost of thespecialized high temperature ovens, and that the process steps offorming the plates and forming the shafts each may require the use of aspecialized process oven for days, a considerable investment of bothtime and money has been invested just to get the overall process to thepoint where the shaft and plate have been completed. Yet a further stepin the specialized process oven is required in present processes toaffix the plate to the shaft. An example of this step would be to jointhe shaft to the plate using a liquid phase sintering step in thespecialized high temperature process oven with a press. This third stepin the specialized process oven also requires significant space in sucha process oven as the assembled configuration of the heater includesboth the length of the shaft and the diameter of the plate. Although themanufacture of just the shafts may take a similar amount of axiallength, the diameter of the shafts is such that multiple shafts may beproduced in parallel in a single process.

As seen in FIG. 4, the joining process to sinter the shaft to the plateagain requires the use of a process oven 160 with a press 161. A set offixturing 164, 165 is used to position the plate 162 and the shaft 163,and to transmit the pressure delivered by the press 161.

Once the heater is completed, it may be used in semiconductorprocessing. The heater is likely to be used in harsh conditions,including corrosive gasses, high temperatures, thermal cycling, and gasplasmas. In addition, the heater may be subject to inadvertent impacts.Should the plate or the shaft become damaged, the opportunities forrepair of a plate and shaft device joined by liquid phase sintering arelimited, perhaps non-existent.

Another prior method for joining ceramic shafts to ceramic platesinvolves the bolting of the shaft to the plate. Such systems are nothermetic even where the adjoining surfaces are polished to enhance thequality of the seal. A constant positive purge gas pressure is requiredinto the inside of the shaft to reduce process gas infiltration.

An improved method for manufacturing semiconductor processing equipmentmay involve the joining of a shaft and a plate, which have beendescribed above, into a final joined assembly without the time consumingand expensive step of an additional liquid phase sintering with hightemperatures and high contact pressures. The shaft and plate may bejoined with a brazing method for joining ceramics. An example of abrazing method for joining together first and second ceramic objects mayinclude the steps of bringing the first and second objects together witha silicon binder, heating the silicon binder to a temperature of atleast 1414 C in vacuum, and cooling the silicon binder to a temperaturebelow its melting point so that the binder hardens and creates ahermetic seal so as to join the first member to the second member. Inother aspects, alloys of silicon may be used in order to reduce, or toslightly reduce, the solidus temperature. Depending upon the processesintended to be run with the completed ceramic piece, in the case ofsemiconductor processing equipment, a silicon alloy may be selectedwhich has a solidus temperature above the process temperature, and whichhas a composition compatible with the processing environment. Variousgeometries of braze joints may be implemented according to methodsdescribed herein.

FIG. 5A shows a cross section of a first embodiment of a joint in whicha first ceramic object, which may be a ceramic shaft 181, for example,may be joined to a second ceramic object, which may be made of the sameor a different material, and which may be a ceramic plate 182, forexample. A braze filler material 180 may be included, which can beselected from the combinations of braze materials or binders describedherein and may be delivered to the joint according to the methodsdescribed herein. With respect to the joint depicted in FIG. 5A, theshaft 181 is positioned such that it abuts the plate, with only thebraze filler interposed between the surfaces to be joined, for exampleend surface 183 of the end 185 of the shaft 181 and an interface surface184 of the plate 182. The thickness of the joint is exaggerated forclarity of illustration.

Although an exemplary embodiment utilized to illustrate the joining ofceramics according to aspects of the present invention is that ofjoining a plate to a shaft, such as may be done when manufacturing aheater or electrostatic chuck used in semiconductor processing, it is tobe understood that the joining of ceramics with silicon according toaspects of the present invention is not limited to such an embodiment.The joining of ceramics with silicon brazing methods as described hereinis itself a novel and useful approach for the joining of ceramics.

FIG. 5B shows a cross section of a second embodiment of a joint in whicha first ceramic object, which may be a ceramic shaft 191, for example,may be joined to a second ceramic object, which may be made of the sameor a different material, and which may be a ceramic plate 192, forexample. A joining material, such as braze filler material 190, may beincluded, which can be selected from the combinations of braze materialsor binders described herein and may be delivered to the joint accordingto the methods described herein. With respect to the joint depicted inFIG. 5B, the shaft 191 is positioned such that it abuts the plate, withonly the braze filler interposed between the surfaces to be joined, forexample surface 193 of the shaft and surface 194 of the plate. Theinterface surface 194 of the plate 192 may reside in a recess 195 in theplate. The thickness of the joint is exaggerated for clarity ofillustration.

The embodiments as illustrated in FIGS. 5A and 5B may include aplurality of standoffs adapted to maintain a minimum braze layerthickness. In some embodiments, as seen in FIG. 5C, the shaft 191 mayutilize a plurality of mesas 171 on the end 172 of the shaft 191 whichis to be joined to the plate. The mesas 171 may be part of the samestructure as the shaft 191, and may be formed by machining awaystructure from the shaft, leaving the mesas. In some embodiments, themesas may be used to create a minimum braze layer thickness of theremainder of the shaft end 172 from the mating surface of the plate. Insome embodiments, the braze filler material, prior to brazing, will bethicker than the distance maintained by the mesas between the shaft endand the plate. With appropriate tolerance control on the interfacesurface of the plate and of the shaft and mesas, the tolerance controlof the finished plate and shaft device may be achieved as the mesas moveto contact the plate interface during the brazing step. In someembodiments, other methods may be used to establish a minimum brazelayer thickness. In some embodiments, ceramic spheres may be used toestablish a minimum braze layer thickness.

As seen in FIG. 6, the brazing material may bridge between two distinctatmospheres, both of which may present significant problems for priorbrazing materials. On an external surface 207 of the semiconductorprocessing equipment, such as a heater 205, the brazing material must becompatible with the processes occurring in, and the environment 201present in, the semiconductor processing chamber 200 in which the heater205 will be used. The heater 205 may have a substrate 206 affixed to atop surface of the plate 203, which is supported by a shaft 204. On aninternal surface 208 of the heater 205, the brazing material must becompatible with a different atmosphere 202, which may be an oxygenatedatmosphere. Prior brazing materials used with ceramics have not beenable to meet both of these criteria. For example, braze elementscontaining copper, silver, or gold may interfere with the latticestructure of the silicon wafer being processed, and are thus notappropriate. However, in the case of a brazed joint joining a heaterplate to a heater shaft, the interior of the shaft typically sees a hightemperature, and has an oxygenated atmosphere within the center of a thehollow shaft. The portion of the braze joint which would be exposed tothis atmosphere will oxidize, and may oxidize into the joint, resultingin a failure of the hermeticity of the joint. In addition to structuralattachment, the joint between the shaft and the plate of these devicesto be used in semiconductor manufacturing must be hermetic in many, ifnot most or all, uses.

A braze material which will be compatible with both of the atmospheresseen on both sides across a joint in such a device is silicon. The brazematerial may be in the form of a sheet, a powder, a thin film, or be ofany other form factor suitable for the brazing processes describedherein. In some embodiments, braze layers which are 0.001 to 0.010inches thick are used. In some embodiments, silicon with a purity ofgreater than 99% is used. In some embodiments, silicon with a purity ofgreater than 98% is used. The solidus temperature of pure silicon isapproximately 1414 C. The maximum practical temperature that silicon canbe exposed to as a joining material is approximately 1625 C, at whichpoint it begins to evaporate at an excessive rate.

In some aspects, “pure” silicon is used as the braze layer. “Pure”silicon, in the language of the technology, may be one of many availablecommercial products referred to as “pure” silicon, but these may be98.6%, 99.0%, or 99.7%, to use some common examples. With regard to thepresent invention, pure silicon may be silicon in excess of 98% purity,or greater than 99% purity. Another aspect of using a braze layer ofsilicon, as opposed to a silicon alloy as discussed below, is that ifthe silicon has nothing mixed within it that results in a compound withSi when brazed, then that is referred to as a non-compounding element.In contrast, if a Si alloy utilizes an element which does compound withSi during brazing, this is referred to as a compounding element andresults in the formation of an Si eutectic or an Si peretactic brazingwhen heated for brazing.

Separate from elements which remain in the assembly after brazing, orwhich form other compounds which remain in the assembly after brazing,there may be materials in the braze layer which are present only priorto the brazing process, and then evaporate or burn off during brazing.As a specific example, a binder may be used to help properly locate a Sipowder to be used as a braze layer. A binder, such as methyl cellulosemixed with a solvent, may be used to locate the Si in the desiredlocation for the braze layer, especially when using a powdered form ofSi. The binder will disappear before the temperatures approach the Sibraze temperatures. When discussing the purity of the silicon brazinglayer, the purity after brazing, after the binder disappears, is what isviewed as the purity of the silicon brazing layer. For example, if asignificant percentage of the pre-braze braze layer material includedsomething such as methyl cellulose, that percentage would not be in thefinal braze layer after heating for brazing.

In some aspects, it may be desirable to get the advantages of usingsilicon as a brazing material but it may be disadvantageous to braze atsuch a high temperature. Some alloys of silicon have solidustemperatures much lower than that of pure silicon. If compatible withlater seen processing chemistries, a variety of alloys may be used toprovide a lower solidus temperature, such that a more practical processoven may be used to effect the braze.

FIG. 7 illustrates a joint 220 used to join a plate 215 to a shaft 214according to some embodiments of the present invention. The joint 220has created a structural and hermetic joint which structurally supportsthe attachment of the plate 215 to the shaft 214. The joint 220 hascreated a hermetic seal which isolates the shaft atmosphere 212 seen bythe interior surface 218 of the shaft 214 from the chamber atmosphere211 seen along the exterior surface 217 of the shaft 214 and within theprocess chamber. The joint 220 may be exposed to both the shaftatmosphere and the chamber atmosphere and must therefore be ablewithstand such exposure without degradation which may result in the lossof the hermetic seal. In this embodiment, the joint may be silicon andthe plate and the shaft may be ceramic such as aluminum nitride. In someembodiments, the joint 220 may be of silicon, and which substantiallyremains in the joint region after the joining process. The residualsilicon may allow for disjoining of the joint for repair, rework, orother reasons.

FIG. 8 shows one embodiment of a schematic illustration of a heatercolumn used in a semiconductor processing chamber. The heater 300, whichmay be a ceramic heater, can include a radio frequency antenna 310, aheater element 320, a shaft 330, a plate 340, and a mounting flange 350.One embodiment of a brazing method for joining together a shaft 330 anda plate 340, both or either one of which may be made of aluminumnitride, to form the heater 300 may be implemented as follows. In someembodiments, a poly-crystalline AlN is used, and is comprised of 96% AlNand 4% Yttria. Such a ceramic may be used in industrial applicationsbecause during the liquid phase sintering used to manufacture theceramic, a lower temperature may be used. The lower temperature process,in contrast to polycrystalline AlN without a sintering aide, reducesmanufacturing costs of the ceramic. The poly-crystalline AlN with addedYttria may also have preferred material properties, such a being lessbrittle. Yttria and other dopants are often used for manufacturabilityand tuning of material properties. With a poly-crystalline AlN such as96% AlN-4% Yttria ceramic, the ceramic presents grains of AlN which areinterspersed with yttrium aluminate. The non-susceptibility of AlN todiffusion with silicon under the conditions of processes according toembodiments of the present invention results in preservation of thematerial properties, and the material identity, of the ceramic after thebrazing step in the manufacturing of the plate and shaft assembly.

A sheet of silicon or silicon alloy metal binder, or powder, or othertype of filler may be provided between the shaft and the plate, and theshaft and the plate may be brought together with the sheet of thesilicon binder disposed therebetween. In some embodiments, the siliconmay be applied with sputtering techniques. In some embodiments, thesilicon may be applied as a powder mixed with a binder, as discussedabove, and painted on. The braze layer may then be heated in a vacuum toa temperature of at least 1414 C (in the case of pure silicon) meltingthe filler material and then cooled allowing the binder or filler tosolidify, creating a hermetic seal joining the shaft to the plate. Theshaft of said heater may be of solid material or it may be hollow inconformation.

In an exemplary embodiment, the plate and shaft may both be of aluminumnitride and both have been separately formed previously using a liquidphase sintering process. The plate may be approximately 9-13 inches indiameter and 0.5 to 0.75 inches thick in some embodiments. The shaft maybe a hollow cylinder which is 5-10 inches long with a wall thickness of0.1 inches. As previously seen in FIG. 5A, the plate 182 may have arecess 185 adapted to receive an outer surface of a first end of theshaft 181. As previously seen in FIG. 5C, mesas may be present on theend of the shaft which abuts the plate. The mesas may be 0.004 incheshigh. The plate 182 and shaft 181 may be fixtured together for a joiningstep with a brazing material 180 of silicon or silicon alloy powderplaced between the pieces along the end of the shaft and within therecess of the plate. The brazing material may be 0.006 inches thickprior to brazing with a completed joint minimum thickness of 0.004inches. The brazing material may be silicon with a purity greater than99%.

Very little physical pressure, other than simple contact, may berequired for joining. The low contact pressures needed for the joiningof the plate to the shaft using the present methods may be able to beprovided using simple fixturing, which may include a mass placed ontothe fixturing using gravity to provide the contact pressure. In someembodiments, contact between the interface portion of the shaft and thebrazing element, as well as contact between the interface portion of theplate and the brazing element, will provide contact pressure sufficientfor joining. Thus, the fixture assembly need not be acted upon by apress separate from the fixture assembly itself. The fixtured assemblymay then be placed in a process oven. The oven may be evacuated to apressure of 1×10E-5 Torr. In some aspects, vacuum is applied to removeresidual oxygen. In some embodiments, a vacuum of lower than 1×10E-4Torr is used. In some embodiments, a vacuum of lower than 1×10E-5 Torris used. Of note with regard to this step is that the high temperatureoven with high contact pressure fixturing, which was required during themanufacture of the ceramic components (shaft and plate), is not neededfor this joining of the shaft and plate. Upon initiating the heatingcycle, the temperature may be raised slowly, to standardizedtemperatures, for example, 600 C and then the joining temperature, andheld at each temperature for a fixed dwell time to allow the vacuum torecover after heating, in order to minimize gradients and/or for otherreasons. When the braze temperature has been reached, the temperaturecan be held for a time to effect the braze reaction. In an exemplaryembodiment, the dwell temperature may be 1425 C and the dwell time maybe 10 minutes. Upon achieving sufficient braze dwell time, the furnacemay be cooled at a rate of 20 C per minute, or lower when the inherentfurnace cooling rate is less, to room temperature. The furnace may bebrought to atmospheric pressure, opened and the brazed assembly may beremoved for inspection, characterization and/or evaluation. In someaspects, the temperature dwell time may be between 1 minute and 1 hour.In some aspects, the temperature dwell time may be between 2 minutes and10 minutes. In some aspects, the dwell temperature may be in the rangeof 1420 to 1460 C.

In some aspects, the brazing element is brought to a temperature abovethe melting (liquidus) temperature under a controlled atmosphere, whichmay be a vacuum. At the desired brazing temperature, the brazing elementthen flows over the substrate surfaces adjoining the filler material(wetting) and forming the basis of the desired joint. A vacuum ambienthelps insure that residual gas existing in the joint region is removedinsuring a more complete wetting of the joint surfaces includinginfusion of the liquid filler into any contours, pores, crevices, andreadily accessible intergranular spaces that may exist in the surfacesof the parts comprising the final joined item.

The wetting and flow of the brazing layer may be sensitive to a varietyof factors. The factors of concern include the braze materialcomposition, the ceramic composition, the composition of the ambientatmosphere during the joining process, which includes the level ofoxygen in the chamber during the joining process, the temperature, thetime at temperature, the thickness of the braze material, the surfacecharacteristics of the material to be joined, the geometry of the piecesto be joined, and the physical pressure applied across the joint duringthe joining process.

In some embodiments, the plate and shaft may comprise differentceramics. The plate may be adapted to provide a high conductive heatcoefficient, whereas the shaft may be adapted to provide a lowerconductive heat coefficient such that heat is not lost down the shafttowards the mounting appurtenances of the process chamber. For example,the plate may be made of aluminum nitride and the shaft may be made ofzirconia.

FIGS. 9-11 illustrate a joining process which may join a shaft to aplate according to some embodiments of the present invention. Thejoining process may be run in a process oven with lower temperatures,contact pressures, and lower time and cost commitments than in previousjoining operations.

In some embodiments, as seen in FIG. 9, alignment and location of theshaft and plate is maintained by part geometries, eliminating fixturingand post-bond machining. Weighting may be used to insure there is nomovement during bonding process, other than some axial movement as thebraze material melts. The plate 400 may be placed top down with ajoining element 402 within a recess 403 in the back surface of the plate400. The shaft 401 may be inserted vertically downward into the recess403 within the plate 400. A weight 404 may be placed on the shaft 401 toprovide some contact pressure during the joining process.

In some embodiments, as seen in FIG. 10, location of the shaft and plateis maintained by part geometries, reducing post-bond machining.Fixturing may be required to maintain perpendicularity between shaft andplate during bond processing. In some embodiments, the tolerancing ofthe mesas and the interface portion of the plate may be used to controlthe dimensions and tolerances of the final assembly. Weighting may alsobe used to insure there is no movement during bonding process, otherthan some axial movement as the braze material melts. The plate 410 maybe placed top down with a joining element 412 within a recess 413 in theback surface of the plate 410. The shaft 411 may be inserted verticallydownward into the recess 413 within the plate 410. A fixture 415 isadapted to support and locate the shaft 411. A weight 414 may be placedon the shaft 411 to provide some contact pressure during the joiningprocess. In some embodiments, a weight is not used. In some embodiments,the mass of the items to be joined may provide force, with gravity, toapply pressure between the items to be joined.

In some embodiments, as seen in FIG. 11, location and perpendicularityof shaft/plate is maintained by fixturing. Fixturing may not be precisedue to thermal expansion and machining tolerances—therefore, post-bondmachining may be required. The shaft diameter may be increased toaccommodate required material removal to meet final dimensionalrequirements. Again, weighting may be used to insure there is nomovement during bonding process, other than some axial movement as thebraze material melts. The plate 420 may be placed top down with ajoining element 422 above the back surface of the plate 420. The shaft421 may be placed onto the plate 420 to create a plate and shaftpre-assembly. A fixture 425 is adapted to support and locate the shaft421. The fixture 425 may be keyed to the plate to provide positionalintegrity. A weight 424 may be placed on the shaft 411 to provide somecontact pressure during the joining process.

An aspect of the current invention is the maximum operating temperatureof the bonded shaft-plate as defined by the decreasing tensile strength,with temperature, of the silicon or silicon alloy selected for thejoining. For example, if pure silicon is employed as the joiningmaterial, the structural strength of the bond between the shaft andplate becomes quite low as the temperature of the joint approaches themelting temperature of the silicon, generally considered to be 1414 C.In practice, a silicon alloy with a lower joint melting temperature maybe sufficient for the processing need. However, some semiconductordevice fabrication processes, or other types of processes supported byequipment using joints according to some embodiments of the presentinvention, may require temperatures as high as 1400 C.

In embodiments using mesas or spheres as standoffs, the originalthickness of the brazing layer prior to the application of heat may beslightly less than, equal to, or larger than the thickness of the finaljoint thickness maintained by the mesas or other devices. As the brazinglayer temperature reaches and exceeds the liquidus temperature, pressureacross the brazing layer between the pieces being joined may causerelative motion between the pieces until the mesas on a first piececontact an interface surface on a second piece. At that point, contactpressure across the joint will no longer be supplied by the externalforce (except as resistance to repulsive forces within the brazinglayer, if any). The mesas, or other standoff technique such as ceramicspheres, may prevent the brazing layer from being forced out of thejoint area prior to the full wetting of ceramic pieces, and may thusallow for better joining. In some embodiments, mesas are not used.

The fixtured assembly may then be placed in a process oven. The oven maybe evacuated to a pressure of less than 5×10E-5 Torr. In some aspects,vacuum removes the residual oxygen. In some embodiments, a vacuum oflower than 1×10E-5 Torr is used. In some embodiments, the fixturedassembly is placed within a zirconium inner chamber which acts as anoxygen attractant, further reducing the residual oxygen which might havefound its way towards the joint during processing. In some embodiments,the process oven is purged and re-filled with pure, dehydrated purenoble gas, such as argon gas, to remove the oxygen. In some embodiments,the process oven is purged and re-filled with purified hydrogen toremove the oxygen.

The fixture assembly is then subjected to increases in temperature, anda hold at the joining temperature. Upon initiating the heating cycle,the temperature may be raised slowly, for example 15C per minute to 200C and then 20 C per minute thereafter, to standardized temperatures, forexample, 1000 C and the joining temperature, and held at eachtemperature for a fixed dwell time to allow the vacuum to recover afterheating, in order to minimize gradients and/or for other reasons. Whenthe braze temperature has been reached, the temperature can be held fora time to effect the braze reaction. In an exemplary embodiment, thedwell temperature may be 1440 C and the dwell time may be 10 minutes.Upon achieving sufficient braze dwell time, the furnace may be cooled ata rate of 20 C per minute, or lower when the inherent furnace coolingrate is less, to room temperature. The furnace may be brought toatmospheric pressure, opened and the brazed assembly may be removed forinspection, characterization and/or evaluation. In another exemplaryembodiment the dwell temperature may be 1425 C.

The brazing material will flow and allow for wetting of the surfaces ofthe ceramic materials being joined. When ceramic such as aluminumnitride is joined using silicon brazing layers and in the presence ofsufficiently low levels of oxygen and described herein, the joint is ahermetic brazed joint.

Joined assemblies joined as described above result in pieces withhermetic sealing between the joined pieces. Such assemblies are thenable to be used where atmosphere isolation is an important aspect in theuse of the assemblies. Further, the portion of the joint which may beexposed to various atmospheres when the joined assemblies are later usedin semi-conductor processing, for example, will not degrade in suchatmospheres, nor will it contaminate the later semi-conductorprocessing.

Both hermetic and non-hermetic joints may join pieces strongly, in thatsignificant force is needed to separate the pieces. However, the factthat a joint is strong is not determinative of whether the jointprovides a hermetic seal. The ability to obtain hermetic joints may berelated to the wetting of the joint. Wetting describes the ability ortendency of a liquid to spread over the surface of another material. Ifthere is insufficient wetting in a brazed joint, there will be areaswhere there is no bonding. If there is enough non-wetted area, then gasmay pass through the joint, causing a leak. Wetting may be affected bythe pressure across the joint at different stages in the melting of thebrazing material. The use of mesa standoffs, or other standoff devicesuch as the insertion of ceramic spheres or powder particles ofappropriate diameter, to limit the compression of the brazing layerbeyond a certain minimum distance may enhance the wetting of the areasof the joint. Careful control of the atmosphere seen by the brazingelement during the joining process may enhance the wetting of the areasof the joint. In combination, careful control of the joint thickness,and careful control of the atmosphere used during the process, mayresult in a complete wetting of the joint interface area that is notable to be achieved with other processes.

Another advantage of the joining method as described herein is thatjoints made according to some embodiments of the present invention mayallow for the disassembly of components, if desired, to repair orreplace one of those two components. Because the joining process mightnot modify the ceramic pieces by diffusion of a joining layer into theceramic, the ceramic pieces are thus possibly able to be re-used.

In an exemplary embodiment, a first ceramic piece of poly-crystallinealuminum nitride is joined to a second piece of poly-crystallinealuminum nitride (AlN) with a silicon brazing layer. As seen if FIGS. 12and 13, the first ceramic piece 501 may be a circular plate of AlN. Thesecond ceramic piece 502 may be a cylindrical ring. The joining of thefirst ceramic piece 501 and the second ceramic piece 502 is illustrativeof the joining of a plate and shaft, such as with a heater, or anelectrostatic chuck, used to support semiconductor processing. A siliconjoining layer 503 has hermetically joined the first piece 501 to thesecond piece 502.

The silicon joining layer 503 is Si powder which is 99.5% Si powder. Thebrazing process was performed in a process oven at a pressure of 1×10E-4Torr. The brazing temperature was 1425 C and was held for 10 minutes.The resulting joint was hermetic. Hermeticity was verified by having avacuum leak rate of <1×10E-9 sccm He/sec; as verified by a standardcommercially available mass spectrometer helium leak detector.

FIG. 14 illustrates a partial cross-section of a plate assemblyaccording to some embodiments of the present invention. A multi-layerplate assembly is another device which may be made with the processesdescribed herein. The plate assembly 240 may be adapted to be joined toa shaft to complete a plate and shaft assembly. The top plate layer 241may be a circular disc adapted to support a substrate duringsemiconductor processing steps. A heater 244 is adapted to reside belowthe top plate layer 241. The heater may be attached or adhered to one orboth of the plate layers. The top plate layer 241 overlays the bottomplate layer 242. A joining layer 243 joins the top plate layer 241 tothe bottom plate 242. The joining layer may be an annular disc. In someembodiments, the top plate layer and the bottom plate layer are ceramic.In some embodiments, the top plate layer and the bottom plate layer arealuminum nitride. In some embodiments the joining layer is silicon. Asevident from the above description, a wide variety of embodiments may beconfigured from the description given herein and additional advantagesand modifications will readily occur to those skilled in the art. Theinvention in its broader aspects is, therefore, not limited to thespecific details and illustrative examples shown and described.Accordingly, departures from such details may be made without departingfrom the spirit or scope of the applicant's general invention.

What is claimed is:
 1. A method for the joining of ceramic materials,said method comprising the steps of: placing a brazing element in ajoint interface area between the first interface surface of a firstceramic piece and the first interface area of a second ceramic piece tocreate a joining pre-assembly, said brazing element comprising silicon;placing the joining pre-assembly into a process chamber; removing oxygenfrom said process chamber prior to the heating step; and heating saidjoining pre-assembly to a first joining temperature, thereby joiningsaid first ceramic piece to said second ceramic piece while maintaininga minimum joint thickness of a first thickness.
 2. The method of claim 1wherein said first ceramic piece comprises aluminum nitride.
 3. Themethod of claim 2 wherein said second ceramic piece comprises aluminumnitride.
 4. The method of claim 1 wherein said brazing element comprisesmore than 98% silicon by weight after brazing.
 5. The method of claim 3wherein said brazing element comprises more than 98% silicon by weightafter brazing.
 6. The method of claim 3 wherein said brazing elementcomprises more than 99% silicon by weight after brazing.
 7. The methodof claim 1 wherein the step of removing oxygen from said process chambercomprises applying a pressure of lower than 1×10E-4 Torr to said processchamber.
 8. The method of claim 3 wherein the step of removing oxygenfrom said process chamber comprises applying a pressure of lower than1×10E-4 Torr to said process chamber.
 9. The method of claim 5 whereinthe step of removing oxygen from said process chamber comprises applyinga pressure of lower than 1×10E-4 Torr to said process chamber.
 10. Themethod of claim 6 wherein the step of removing oxygen from said processchamber comprises applying a pressure of lower than 1×10E-4 Torr to saidprocess chamber.
 11. The method of claim 5 wherein said step of heatingsaid joining pre-assembly to a first joining temperature comprisesheating said joining pre-assembly to a temperature between 1410 C and1625 C.
 12. The method of claim 6 wherein said step of heating saidjoining pre-assembly to a first joining temperature comprises heatingsaid joining pre-assembly to a temperature between 1410 C and 1625 C.13. The method of claim 9 wherein said step of heating said joiningpre-assembly to a first joining temperature comprises heating saidjoining pre-assembly to a temperature between 1410 C and 1625 C.
 14. Themethod of claim 10 wherein said step of heating said joiningpre-assembly to a first joining temperature comprises heating saidjoining pre-assembly to a temperature between 1410 C and 1625 C.
 15. Themethod of claim 5 wherein the step of removing oxygen from said processchamber comprises purging and re-filling the chamber with pure,dehydrated inert gas.
 16. The method of claim 6 wherein the step ofremoving oxygen from said process chamber comprises purging andre-filling the chamber with pure, dehydrated inert gas.
 17. The methodof claim 4 wherein the step of removing oxygen from said process chambercomprises applying a pressure of lower than 1×10E-4 Torr to said processchamber.